Recent findings from several laboratories suggest that differential cytokine gene expression in T cells is epigenetically enforced through differences in cytokine gene chromatin structure and methylation. However, the role of DNA methylation in this process in vivo has not been tested directly. We have available mice in which the major DNA methyltransferase gene (Dnmtl) is selectively and efficiently ablated during T cell development at the CD4+CD8+ (DP) stage (CD4 Cre Dnmt 21ox mice). T cell development in these mice is essentially normal. We propose to use these mice to determine the role of DNA methylation in the regulation of cytokine gene expression. Specifically, we will address the following hypotheses: First, naive CD4 and CD8 T cells lacking Dnmt 1 will produce effector cytokines more readily (requiring fewer cell divisions) after primary activation than naive T cells from control mice. Second, differences in the pattern of cytokines expressed by Dnmt-/- T cells will be less evident than in control T cells. Third, the loss of Dnmt will result in a more open chromatin structure surrounding cytokine gene loci. Fourth, CD4 and CD8 T cells from CD4 Cre Dnmt 21ox mice will be unable to develop and maintain a heritable, polarized type 1 vs. type 2 pattern of cytokine gene expression. Lastly, CD4 Cre Dnmt 21ox mice will favor biallelic over monoallelic expression of cytokine genes.